The stopping distance d of a car after the brakes are applied varies directly as the square of the speed r. If a car travelling 80 mph can stop in 400 ft, how many feet will it take the same car to stop when it is travelling 64 mph?

Explanation:

The displacement of car is given as directly proportional to its speed

Thus ( 80 )² ∝ 400                          I

Similarly in second case

( 64 )² ∝ S                                      II

dividing II by I

=

here we are to find the S displacement

Thus S = 256 ft

#1

Jill and Scott both moves for 30 minutes

now if Jill cover 5 km distance and Scott cover 10 km distance

now we know that the formula of speed is given as

now we will have

speed of Jill

speed of Scott

so correct answer here is

Scott had the faster speed since he rode at 20 k/h while Jill only traveled 10 km/h.

#2

distance travelled by each car is given as

now here it is given that

time taken by green car

time taken by yellow car

now we can find the speed of two cars

speed of green car

speed of yellow car

so correct answer will be

The yellow car was faster.  Yellow traveled at a speed of 50 mph while green was traveling at an average of 40 mph.